And I recently notice that it takes 4ms for my watchdog timer to synchronize new settings. It would be quite amount of time if MCU waking up from sleep mode frequently.

Now, I wander what if I do not wait WDT sync. Can I still reset it when it is synchronizing? Or else, what the worst might happen if I skip wait for WDT_SYNCBUSY()?

"The life of the individual only has meaning insofar as it aids in making the life of every living thing nobler and more beautiful. Life is sacred, that is to say, it is the supreme value to which all other values are subordinate."-- Albert Einstein